The present invention relates to a process for purifying caustic soda by separating off sodium hydroxide in the form of crystalline NaOH.3.5H.sub.2 O. In particular, the invention relates to a process for isolating particularly 63 pure crystalline NaOH.3.5H.sub.2 O from a crude sodium hydroxide solution containing soluble impurities such as sodium chloride or sodium chlorate, as is obtained from the electrolytic diaphragm process.
The industrial production of NaOH by electrolytic decomposition of an aqueous sodium chloride solution (chlor-alkali electrolysis) is nowadays carried out predominantly by two processes. In the amalgam process, use is made of a mercury cathode on which the sodium metal is deposited to form an amalgam. Subsequent decomposition of the sodium amalgam with water gives an essentially chloride-free sodium hydroxide solution. A problem in this process is the use of mercury as cathode material, which requires particular measures for preventing environmental pollution by mercury.
In addition, the diaphragm process is used on a large scale. In this process the electrolysis cell is divided into anode and cathode compartments by a porous diaphragm. In the cathode compartment, liberation of hydrogen at a steel cathode gives a solution having an NaOH content of customarily from 130 to 150 g/l and an NaCl content of from 175 to 210 g/l as cathode liquor or cell liquor. This cell liquor is subsequently concentrated with precipitation of salt to give a 50% by weight strength sodium hydroxide solution. This leaves a high residual sodium chloride content of about 1% by weight in the alkali. Furthermore, components of the alkali solution formed in the cathode compartment during the electrolysis can diffuse through the porous diaphragm into the anode compartment and react with the chlorine gas formed at the anode to give sodium hypochlorite. Sodium hypochlorite formed in this way subsequently decomposes by disproportionation into sodium chloride and sodium chlorate, as a result of which the caustic soda obtained in the diaphragm process can also contain up to 1.6 g/kg of sodium chlorate as impurity. However, this high content of corrosive impurities interferes in many applications.
A known method of purifying diaphragm caustic soda is the extraction of sodium chloride with liquid ammonia, which allows the NaCl content of the alkali to be reduced to about 0.08% by weight. This process has high capital and operating costs.
The purification of diaphragm alkali is preferably carried out by separating off NaOH in the form of crystalline sodium hydroxide hydrates. DE-A 26 18 242 describes a process for purifying caustic soda by separating off sodium hydroxide as sodium hydroxide dihydrate (NaOH.2H.sub.2 O). Cooling an NaCl-containing, about 50% by weight strength crude sodium hydroxide solution forms a slurry comprising NaOH.2H.sub.2 O crystals and fine NaCl crystals. From this slurry, the fine impurity crystals are removed by flotation with adsorption onto bubbles which are formed by vaporization of a dissolved coolant or by passing a gas into the slurry. Owing to the necessity of separating the sodium hydroxide dihydrate crystals from sodium chloride crystals which are precipitated simultaneously, this purification process is technically complicated and, in addition, requires the undesirable use of CFCs, or instead the use of flammable gases such as butane which requires strict safety precautions, as coolant and flotation agent.
In further known methods of purifying NaOH, sodium hydroxide is separated off as the 3.5-hydrate (NaOH.3.5H.sub.2 O), avoiding a coprecipitation of NaCl. Crystalline NaOH.3.5H.sub.2 O is stable in the range from 5.degree. C. to 15.5.degree. C. in NaOH solution having a concentration of from 33% to 45% by weight. In the absence of seed crystals, such a 33-45% by weight strength NaOH solution can be supercooled to up to 20.degree. C. below its saturation point without crystallization occurring. The crystallization of NaOH.3.5H.sub.2 O from such a supersaturated solution is usually induced by means of seed crystals. After precipitation, the crystallized material can be separated from the mother liquor of the crystallization by means of a mechanical solid/liquid separation.
DE-A-24 26 282 describes a process in which a 33-45% by weight strength aqueous NaOH solution which has been obtained by concentrating the cell liquor obtained in the diaphragm process is cooled to below the saturation point of NaOH.3.5H.sub.2 O and crystalline NaCl which precipitates is separated off. NaOH.3.5H.sub.2 O is crystallized from the homogeneous supersaturated NaOH solution thus obtained by introducing NaOH.3.5H.sub.2 O seed crystals without further external cooling. During this procedure, the mixture is warmed to the saturation temperature of the solution in respect of NaOH.3.5H.sub.2 Oby the heat of crystallization or by external heating. The crystalline NaOH.3.5H.sub.2 O which precipitates is finally, when the saturation temperature is reached, separated off by means of a centrifugal separator.
DE-A-24 34 447 describes a variant of this process in which the crystallization of NaOH.3.5H.sub.2 O is carried out at constant temperature with removal of the heat of crystallization.
The last two processes achieved NaCl contents of less than 1.5 g per kg of NaOH.3.5H.sub.2 O, the minimum NaCl content was 400 mg/kg. No statements are made regarding the content of other impurities such as sodium chlorate.
A disadvantage of both processes is the NaCl content of the caustic soda obtained which is still high in comparison with caustic soda from the amalgam process.